a NON 4.0 RS, I believe even the "regular" GT3 will run faster times than the M3 GTS

Yes this is exactly correct! Even a 2007 997.1 GT3 will beat an m3 gts decisively even though it has less hp less tq and no dual clutch transmission.
Any other GT3 or newer model isn't worth even worth discussion because the gts can't compare to the high limits, and overall performance.
In all honestly, it needs to be a 458 italia or a Lamborghini lp or mclaren to even begin to compare to a GT3 iteration and even those can't match it.